Bit of a guess, but I would have the doors naturally held open, probably by a spring arm, and closed by the wheel as it enters the bay.

I did fit doors once on my B25's nose wheel, using an over center spring, (rubber band), to flip them open as the wheel pushed against them, but they gave so much trouble, occasionally shutting before the wheel retracted, that I eventually took them off.

On a much bigger model they would have separate servos driving the doors, probably from some sort of timed electronic sequencer. That way the wheels would be fully in before the doors close. But I'm not a scale buff.

The hinge would have to be an over-center type, (have a look at those small boxes where the lid snaps shut, and springs open once past the half way. Often on small jewelery boxes. Or make something similar).

Then run a cord between the door and the wheel well, (nylon fishing line would be almost invisible), so that the wheel hits the cord and pulls the door shut. Could be tricky to get the length right, but should work.

It's about the simplest way I can think of so far.

Edit - Forgot to say that the wheel opens the door. Once it reaches so far the over-center hinge springs it open.

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The over-center spring is a good design. But... If you are building the MM/LA P-51, there will be some issues with an over-center spring, due to the amount of space you have to work with and servo torque.

An over-center spring has two relaxed points, on either side of the center tension. You would have to overcome the center tension with your gear. Remember that the gear is about five inches long, driven by a small, sub-micro servo. Many people have a great deal of trouble getting the servo to not 'buzz' due to 'any' added load, even when fully open or closed. Now, if you try to push past the center tension area of an over-center spring, the gear may not open, or close due to the torque needed to overcome the spring at nearly five inches from the pivot point of the gear mechanism. The spring tension would have to be so slight, for the gear to overcome it, that the door would easily open or close by itself. The slightest breeze or roll, would be enough to do it. If this is the case, then you may wind up in a situation where the gear door may be closed, when you need to retract your main gear. Or, one will be closed and one will be open. Either way... you will need to use a system where the gear door will be stable in its open or closed position and does not add any more torque requirement to the servo.

The best way to do this is to use the E arm, a 4g servo with an stripped down EMS sequencer. In fact, talking to you about this, has convinced me to do the very same thing on mine. I don't think that a 4g servo with a stripped down EMS sequencer is going to add any significant weight.